The present invention relates to a technology for conducting cleaning of a fixing device for fixing a toner image onto the sheet in the image forming apparatus, such as a printer, a copier and a facsimile machine of an electro-photographic system.
An image forming apparatus of an electro-photographic system transfers a toner image onto a sheet and the toner image is fixed onto the sheet by heating with the fixing device.
On this image forming process, there is a case that a sheet jam is generated as a sheet gets stuck in the fixing device. FIG. 9 illustrates the situation of occurrence of sheet jam in the fixing device. FIG. 9 illustrates a schematic cross sectional view showing the sheet P, which generates the jam in the printer section in the image forming apparatus and in the fixing section of the printer.
As illustrated in FIG. 9, a printer section 40 includes an image forming section 41 and a fixing device 42. The image forming section 41 includes photoreceptor drums 51Y, 51M, 51C and 51K and an intermediate transfer belt 43, which is stretched around rollers. A secondary transfer member 45 conducts a secondary transfer of the toner image onto the sheet in the secondary transfer position D. The fixing device 42 includes a fixing member and a pressing member. The fixing member is configured by a fixing roller 421a, a stretching roller 421b and a fixing belt 421 strained. A pressing roller 423 is utilized as a pressing member.
The image forming apparatus forms an electrostatic latent image on the photoreceptor drums 51Y, 51M, 51C and 51K by irradiating a laser beams thereon and forms a toner image by developing the electrostatic latent image using developer. Then, this toner image will be transferred (a primary transfer) onto the intermediate transfer belt 43. The toner image on the intermediate transfer belt 43 arrives at the secondary transfer position D by a drive of the intermediate transfer belt 43. The toner image arrived at the secondary transfer position D will be transferred (the secondary transfer) onto a sheet P. The toner image, which has been transferred onto the sheet P but has not been fixed yet, is transferred to the fixing device 42 and the toner image is fixed onto the sheet P with heat and pressure. After that, the sheet P, onto which the image has been formed, will be ejected from the apparatus.
On the image forming process described above, as illustrated in FIG. 9, there is a case that the sheet P is wound in the fixing device 42 and sheet jam occurs. In case when the sheet P is stopped by the sheet jam under a fixing process, there is a case that the toner, which has been fixed once onto the sheet, is melted by the heat of the fixing belt 421, retransferred and fixed onto the fixing belt 421. The residual toner remaining on the fixing belt 421 becomes toner dirt of the fixing device 42.
This residual toner remains fixed onto the fixing belt 421, and arrives at the nip portion of the fixing belt 421 and a pressing roller 423 by the rotation of the fixing belt 421. Then the heat of the fixing device 42 melts the residual toner. The melted toner will be fixed onto the sheet, which is processed in the next operation and generates the toner dirt on the image.
In order to resolve the toner dirt, cleaning of the fixing device is necessary. However, the temperature of the fixing device is very high and there is a case that the cleaning by an operator is at risk.
Thus, an image forming apparatus having a fixing device with a cleaning mechanism, such as a cleaning blade, has been proposed. However, in case that the cleaning mechanism has been provided in the fixing device, the fixing belt is worn out and deteriorated due to the cleaning by the cleaning mechanism, and there is a possibility that belt durability is lost. Further, in case when the cleaning mechanism is provided in the image forming apparatus, the manufacturing process of the image forming apparatus becomes complicated and the cost of the image forming apparatus increases.
Thus, the cleaning of the fixing device needs to be safe without newly providing the cleaning mechanism. From this viewpoint, in the conventional image forming apparatus as illustrated in FIG. 10, a cleaning mode has been provided for cleaning a fixing device by sheet feeding a white sheet of paper. Here, the cleaning mode in the conventional image forming apparatus will be described by using FIG. 10.
FIG. 10 illustrates a schematic cross sectional view showing a situation where a sheet feeding is conducted by a cleaning mode executed in the printer in the conventional image forming apparatus. As illustrated in FIG. 10, a blank image is formed on the photoreceptive drums 51Y, 51M, 51C and 51K without forming a toner image, so that the no toner image is transferred to a sheet at the secondary transfer position D. Based on this arrangement, the sheet P will be conveyed to the fixing device 42 in a blank sheet state, onto where the toner image has not been transferred at the secondary transfer position D.
Next, the fixing device 42 executes fixing same as the normal image formation, against the sheet P having arrived to the nip portion. In this case, the residual toner adhered onto the fixing belt 421 is melted at the nip portion and retransferred and fixed onto the blank sheet P. As a result, the force to be adhered to the sheet P becomes larger than the force to be adhered onto the fixing belt 421. Further, an exfoliation force due to the stiffness force of the sheet P is added to the force to be adhered to the sheet P and the toner exfoliates from the fixing belt 421. Thus, as illustrated in FIG. 10, the cleaning of the toner dirt on the fixing belt 421 can be executed. This technology has been disclosed in Unexamined Japanese Patent Application Publication No. 2003-274151.
However, according to the sheet feed based on the execution of the cleaning mode in the image forming apparatus disclosed in Unexamined Japanese Patent Application Publication No. 2003-274151, as illustrated in FIG. 9, there is a case that the leading edge of the sheet P is adhered onto the fixing belt 421 due to the melted toner having a large adhesiveness and be wound in the fixing belt 421. In this case, sheet jam due to the residual toner occurs. The situation of the sheet jam due to the residual toner will be explained by utilizing FIG. 11. FIG. 11 illustrates a schematic cross sectional view wound by the printer section of the image forming apparatus and the fixing device of the printer.
Namely, normally, even when the melted toner, as illustrated in FIG. 10, is adhered onto both of the fixing belt 421 and the sheet P, based on the adhesive force onto the sheet P and the exfoliation force due to the stiffness of the sheet P itself, the toner exfoliates from the fixing belt 421 and the sheet P is conveyed into the conveyance direction (x direction in FIG. 10). However, as illustrated in FIG. 11, in case when the melted residual toner is adhered onto the leading edge of the sheet P, which is wound to the fixing belt 421 through the residual toner causing sheet jam as illustrated in FIG. 9.
In case when sheet jam occurs in the sheet feed of the cleaning mode, the toner dirt of the fixing device is not resolved and the sheet feed needs to be executed again, which makes the cleaning mode complicated. Further, the efficiency of the sheet feed by the cleaning mode decreases.
With respect to the sheet jam in the sheet feed in a conventional cleaning mode, in order that the sheet is not wound on the fixing belt 421, it is thought that a dedicated cleaning sheet be used to avoid the sheet from being wound. However, every time when executing a cleaning mode, the dedicated cleaning sheet has to be set, which makes the cleaning of the fixing device complicated.